The present invention relates to fuel cells which operates at room temperature used in portable power sources, power sources for electric vehicles, household cogeneration systems and the like. Further, the present invention relates to a solid polymer electrolyte fuel cell and a method for producing the same, in particular.
The solid polymer electrolyte (hereinafter, referred to simply as xe2x80x9cpolymer electrolytexe2x80x9d) fuel cells allow a fuel gas such as hydrogen to electrochemically react with an oxidization agent gas such as air at their gas-diffusion electrode, thereby to simultaneously generates electricity as well as heat.
An example of this kind of the polymer electrolyte fuel cells is illustrated in FIG. 2.
On both surfaces of a polymer electrolyte film (hereinafter, referred to simply as xe2x80x9celectrolyte filmxe2x80x9d) 3, which selectively transports hydrogen ions, catalytic reaction layers 2 consisting mainly of carbon powder carrying a platinum group metal catalyst are arranged in close contact with the film. On the surfaces of the catalytic reaction layers 2, a pair of diffusion layers 1 having both of gas-permeability and electrical conductivity are additionally arranged while being in close contact. An electrode 23 is configured with this diffusion layer 1 and the catalytic reaction layer 2.
On the surfaces of the electrode 23, there are arranged electrically conductive separator 4 in the form of plate for mechanically fixing an electrode-electrolyte assembly 22 composed of these electrodes 23 and electrolyte film 3 (hereinafter, referred to as xe2x80x9cMEAxe2x80x9d) and electrically connecting the adjacent MEAs 22 with each other in series. On the surfaces of the separators 4 in contact with the electrode 23, there are formed gas-flow paths 5 for supplying the reactive gases to the electrodes and exhausting gases generated by the reaction or residual gases. The gas-flow paths 5 may be provided independent of the separator 4, but it is general to provide grooves on the surfaces of separator 4 as the gas-flow paths.
In order to supply the fuel gas to the grooves, it is required to branch a pipeline for supplying the fuel gas into the separators in their numbers and to connect the ends of the branched pipelines directly to the grooves of the separators by means of a piping jig. This jig is referred to as xe2x80x9cmanifoldxe2x80x9d, and one of the type that connects the pipelines of the fuel gas directly to the grooves is referred to as xe2x80x9couter manifoldxe2x80x9d.
On the other hand, there is also the other type called xe2x80x9cinner manifoldxe2x80x9d whose structure is simple. The inner manifold is configured by providing ports on the separator already having the gas-flow paths and allowing the inlets and outlets of the gas-flow paths to reach to the ports. The fuel gas is supplied through the ports.
On the other surfaces of the separators 4 which are placed in every two cells and not in contact with the MEAs, there are provided coolant-flow paths 24 for distributing cooling water employed for maintaining the cell temperature constant. By distributing the cooling water, thermal energy generated by the reaction may be recovered and utilized in the form of hot or warmed water.
In addition, in order to prevent hydrogen and/or air from leaking outside of the cell or mixing with each other, and in order to prevent the cooling water from leaking outside of the cell, sealants 17 that put the electrolyte film 3 therebetween or O-rings 18 are arranged around the circumference of the electrodes 23. There is also such a case wherein these sealants 17 and O-rings 18 have previously been assembled by combining them with the electrodes 23 and electrolyte films 3 in an integral body.
As another method for the sealing, there is such a structure as shown in FIG. 3, wherein a gasket 19 of a resin or a metal having a thickness of substantially the same as that of the electrode is arranged around the circumference of the electrode and the gaps between the gasket 19 and the separators 4 are sealed with the sealant 17 such as grease or an adhesive.
In recent years, there is proposed an alternative method as shown in FIG. 4, wherein the MEAs configured with the electrodes 23 of the same size as that of the electrolyte film 3 are used. And, a resin 21 which has a sealing effect has previously been impregnated into the portions where the gas-tight sealing are required, thereby to allow the resin to solidify therein. That is, the method of securing the gas-sealing property between the separators 4 by impregnating the resin 21 is devised.
As previously described, many of the fuel cells employ a laminated structure configured by stacking a number of unit cells. In order to exhaust heat generated by the electric power during the fuel cell operation to the outside of the cells, cooling plates are arranged in every 1 to 3 unit cells of the laminated cell. As the cooling plates, one that has such a structure wherein a thermal medium such as cooling water is distributed through a space surrounded by metallic plates is generally employed. As shown in FIG.2 to FIG.4, the coolant-flow paths 24 are formed on the rear face of the separator 4, i.e. the surface where the cooling water flows through, thereby to allow the separator 4 itself to function as the cooling plate. In this structure, the O-rings or the gaskets are required for sealing the thermal medium such as cooling water, but in this sealing method, it is necessary to secure a satisfactory electrical conductivity between the top and bottom surfaces of the cooling plates by, for instance, completely pressurizing and deforming the O-rings.
Then, as regards the previously-described manifold, the inner manifold type is generally used that have the gas-supply ports and the gas-exhaust ports for the respective unit cells as well as the supply/exhaust ports for the cooling water inside of the laminated cell. Herein, as an example of the polymer electrolyte fuel cells of the inner manifold type, a partly cut-out perspective view thereof is illustrated in FIG. 5.
As the same as the structure shown in FIG. 2, the diffusion layers 1, the catalytic reaction layers 2, the electrolyte films 3 and separators 4 are laminated, and the gas-flow paths 5 are formed. And, the gas manifolds 8 for supplying/exhausting the gas to/from the cells as well as the cooling water manifolds 8xe2x80x2 for supplying/exhausting the water for cooling the cell are also formed in the laminated structure.
In the case of operating the cell of such inner manifold-type by the use of a reformed gas, however, the electrode is poisoned to decrease the temperature of the cell as the result of increase in the concentration of carbon monoxide at the down streams of the fuel gas-flow paths in the respective unit cells. And the decrease in the temperature further facilitates the electrodes to be poisoned.
In order to suppress such decrease in the cell performance, the outer manifold-type is now also attracting attention again whose structure capable of securing a width of the gas supplying and exhausting portions from the manifold to the respective unit cells as largely as possible.
In the case of the fuel cells of the inner manifold type, the reliability on the gas-sealing property is high in general because a squeezing or tightening (binding) pressure is constantly added onto the whole cell structure. In contrast, in the case of the fuel cells of the outer manifold type, it is relatively hard to obtain an even and flat sealing face because the flanks (side faces) of the laminated unit cells that are in contact with the flange of the manifold are a laminated body composed of the thin sheets such as MEAs and separators. That is, the outer manifold type in general has a lower reliability as compared with the inner manifold type.
In the case of the inner manifold type, however, when the lamination number and the output power of the fuel cell are increased, a large quantity of fluid must be supplied and exhausted through the ports of the inner manifold. Thus, a pressure loss in the manifold increases. It is, therefore, required to make the manifold ports have a smaller diameter in the fuel cell which has a small lamination number by considering the compactness of the whole cells, and conversely, to make the manifold ports have a larger diameter in the fuel cell which has a large lamination number in order to suppress the pressure loss. For that reason, in the inner manifold system, there has been a problem that the lamination number should be considered in the design of the separators and the MEAs.
In any case, it is required to stack a number of the unit cells with cooling portions up in one direction, to arrange a pair of end plates on the both end of the stacked body, and to fix the stacked body by connecting the two end plates with binding rods. As a system for squeezing, it is desirable to squeeze the unit cells as uniformly as possible with respect to their planes, and it is usual to use a metallic material such as stainless steel or the like for the end plates and the binding rods in viewpoint of mechanical strength. These end plates and binding rods are electrically insulated from the laminated unit cells by insulation plates or the like and used in a structure whereby any current will not leak outside through the end plates. As regards the binding rods, there are proposed a modified system whereby the binding rods are guided through ports provided on the separator plates and another system of squeezing the laminated unit cells as a whole with metal belts over the end plates.
In addition, in any of the sealing methods shown in FIG. 2 to FIG. 4, it is necessary to apply a constant pressure for maintaining the sealing property and keeping the contact resistance between the electrodes and the separators small and, thus, such a configuration is employed wherein screw springs or dish washer is inserted between the binding rods and the end plates. With this squeezing pressure, the electrical contact between the members such as separators, electrodes, electrolyte films and the like is also secured.
On the other hand, the separators in the described polymer electrolyte fuel cells are required to have a high electrical conductivity, a high gas-tightness to the fuel gas and a high corrosion resistance against the reaction product generated at the time of oxidizing/reducing of hydrogen and oxygen. For these reason, the conventional separators are configured with a carbon material such as glassy carbon, expanded graphite or the like. The gas-flow paths are formed by means of machine tool cutting and, in the case of the expanded graphite, by molding with a die.
In recent years, however, plates of metal such as stainless steel or the like have been used in place of the conventional carbon material. FIG. 6 and FIG. 7 show schematic plan views of a conventionally used separator, respectively. As shown by FIG. 6, by providing ribs 6 of a resin material or the like on the circumferential parts of the separator 4 and around the circumferences of the ports of the inner manifolds and by inserting, for instance, a metallic mesh 7 or a corrugated fin between the electrode and the metallic plate, the gas-flow paths are formed. Alternatively, as shown by FIG. 7, one system may be considered that by pressing a metallic plate to have concaved portions 9 and convex portions 10, the gas-flow path is formed with the concaved portions 9 which connects the ports of the gas supply side with the ports of the gas exhaust side in the manifold.
For the fuel cells illustrated in the above, there is a need for arranging the sealants or the O-rings around the circumference of the electrode for preventing the gas such as hydrogen or air from leaking. At that time, the MEA is required to have a margin as wide as about 10 mm for providing the sealant. In a method of allowing a resin having a sealing effect to impregnate into the MEA to provide the sealing portion, a margin as wide as about 0.5 mm is also required.
For realizing a smaller size, a more compactness and a further reduction in the occupied space of the fuel cell, it is required to make these margins for the sealing as small as possible. In addition, the sealants or the sealing portions are required to be sandwiched by separators from the top and bottom, and a relatively large squeezing pressure must be constantly applied. For that reason, the sizes and weights of the squeezing jigs such as the end plates and the binding rods increase, and are the problems for realizing the compactness and the lighter weight of the whole cells.
According to the method of using the sealants or the O-rings, or the method of sealing by impregnating the resin into the MEA, there is a need for the number of the steps for the sealing and, thus, a further inventive means has been desired. In addition, although a satisfactory pressure is required to be maintained between the electrolyte films and the electrodes, and between the electrodes and the separators in the polymer electrolyte fuel cells, there is also a problem that it is difficult to control the thickness for applying the satisfactory pressure because the squeezing pressure between both end plates also acts on the both of the electrodes and the sealing portions.
For the fuel cells of the inner manifold type having a small lamination number, it is required to make the ports of the manifold have a small diameter by considering the compactness of the obtained fuel cell as a whole. On the other hand, for the fuel cells having a large lamination number, it is required to make the ports of the manifold have a large diameter in order to suppress or reduce the pressure loss. That is, there is a problem that in designing the separator and the MEA, the lamination number must be a constant concern.
In view of the above-mentioned unsolved problems, the present invention has an object of providing a polymer electrolyte fuel cell which has an excellent gas-sealing property between the above-mentioned manifold and the surface of the module of the laminated unit cells. The present invention also has another object of providing a method for producing the same.
According to the conventional method of producing the separator plate by means of cutting the carbon plate, it is difficult to reduce the material cost of the carbon plate and the cost for cutting the plate. According to the method of using the expanded graphite, the material cost is also expensive. And, thus, these methods are believed to be an obstacle to putting them into practice.
Alternatively, according to the above-mentioned method of using the metallic plate in the case of the separator shown in FIG. 6, when the amount of the flowing gas is reduced and the utilization of the gas is raised for the purpose of reducing the energy required for supplying the gas, the flow rate of the fuel gas at the surface of the electrode becomes small. Thus, the exhaustion of the produced water vapor is made difficult. In addition, it is difficult to cause the gas to flow uniformly between the supply side and the exhaust side of the manifold.
According to the separator illustrated in FIG. 7, it is however possible to maintain the gas-flow rate and to flow the gas uniformly. In this structure, however, the sealing for the fuel gas is difficult because the end faces of the manifold inevitably is in corrugated structure. In addition, there is a number of limitations on the processing of the metallic plate, and it is difficult to design the gas-flow path free.
It is therefore an object of the present invention to also provide a separator that can solve these problems.
The present invention provides a solid polymer electrolyte fuel cell comprising a plurality of unit cells laminated while placing electrically conductive separators therebetween, said unit cell comprising an electrode having a pair of catalytic reaction layers which sandwich a solid polymer electrolyte film therebetween, a means for supplying and distributing a fuel gas mixture containing hydrogen to one surface of the above-mentioned electrode, and a means for supplying and distributing an oxidant gas containing oxygen to the other surface of the above-mentioned electrode, wherein gas-tight and electrically insulating layers are provided on the flanks or the insides of the laminated unit cells, thereby to give a gas-tightness between the electrode and separator.
It is preferable that the electrically insulating layers comprise a composite material configured with the above-mentioned electrically insulating material and a material having a larger tensile strength than the insulating material, for the covering purpose.
It is also preferable to provide a gas manifold on the flanks of the module of the above-mentioned laminated unit cells via the above-mentioned electrically insulating layer, thereby to supply and distribute the fuel gas to the unit cell module.
In addition, it is preferable that the edges of the electrodes of said unit cells reach the flanks of said module of the laminated unit cells.
Further, it is preferable that the above-mentioned gas manifold is composed of an elastic material.
In addition, it is preferable that the material for constituting the above-mentioned gas manifold is the same as that consituting the above-mentioned electrically insulating layers.
Further, it is also preferable that the above-mentioned electrically insulating layers comprise a resin or a rubber.
In addition, it is preferable that the above-mentioned separator is composed of a metallic plate having a gas-distributing groove, and that the above-mentioned gas-distributing groove is connected to the means for supplying and distributing the above-mentioned fuel gas by a gas-tight material.
Further, it is also preferable that the above-mentioned gas-distributing groove is composed of a plurality of linear grooves disposed in parallel with each other.
In addition, it is preferable that the above-mentioned gas-distributing groove formed on one surface of the metallic plate forms a concave part for a gas-distributing groove on the other surface of said metallic plate.
Further, it is also preferable that the above-mentioned separator is composed of a plurality of metallic plates and that a gas-distributing groove is provided on the entire surface of at least one of the metallic plates.
In addition, it is preferable that when the above-mentioned gas-tight non-metallic material is pressed against the metallic plate with a pressure of not less than a given value, the surfaces of the metallic plate in contact with the gas-tight non-metallic material have a gas-tightness to said fuel gas.
In addition, the present invention also provide a method for producing a solid polymer electrolyte fuel cell comprising a plurality of unit cells laminated while placing electrically conductive separators therebetween, said unit cell comprising an electrode having a pair of catalytic reaction layers which sandwich a solid polymer electrolyte film therebetween, a means for supplying and distributing a fuel gas mixture containing hydrogen to one surface of the above-mentioned electrode, and a means for supplying and distributing an oxidant gas containing oxygen to the other surface of the above-mentioned electrode, comprising the steps of:
laminating a plurality of the unit cells via separators therebetween and mechanically binding and fixing the plurality of the unit cells from the both ends to produce the laminated unit cells,
forming sealing portion comprising a gas-tight and electrically insulating material or a composite of said electrically insulating material and a material having a larger tensile strength than the electrically insulating material on the flanks of the laminated unit cells, and
providing gas manifolds on the above-mentioned laminated unit cells via the sealing portions to join the above-mentioned sealing portions on the above-mentioned manifolds gas-tightly.
In the case of this method, it is preferable to join the above- mentioned sealing portions on the gas manifolds by means of ultrasonic welding.
Further, by molding the above-mentioned laminated unit cells and the gas manifold into an integral body by means of injection molding process, the gas manifolds may be provided on the laminated unit cells via the sealing portions between them to join said sealing portions on said manifolds gas-tightly.
In addition, in the above-mentioned separator, the gas-distributing grooves on the separator may be formed by means of pressing or folding process on a metallic plate.
Further, the above-mentioned separator may also be produced by first forming a corrugated pattern on the entire surface of a metallic plate by means of pressing or folding to form the gas-distributing groove having a plurality of linear grooves disposed in parallel with each other and, then, by flattening a part of the above-mentioned metallic plate having the corrugated pattern.
In addition, it is also possible to integrate the gas-sealing material with the metallic plate having the gas-distributing grooves by an adhesion or glazing of the material on the metallic plate.
While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings.